Femto cells or femto cell access points (FAP) are small cellular base stations designed for extending coverage of wireless communication networks. Each femto cell provides services to users registered to that particular FAP. FAPs use broadband connectivity in order to connect to the operator network.
All base stations, including FAPs, broadcast certain basic information such as location area code (LAC) in the form of system information blocks, which is used by user equipment (UE) to identify and access the base station.
A UE tries to perform cell reselection by evaluating serving cell and neighbor cell measurements and by reading the system information blocks broadcasted by serving cell and neighbor cell. Within the context of a femto radio access network (RAN), during the cell reselection process, the UE may encounter a “closed access” neighbor cell in which it is prohibited to access. A UE that is older than 3gPP (3rd generation partnership project) release 8 or a UE that is not closed subscriber group (CSG) capable would not be femto aware and hence would not be aware that the UE is unauthorized to use the closed access neighbor cell. When the FAP receives a connection request from an unknown UE, the FAP attempts to identify the UE. If the UE is unauthorized, the FAP sends a non-access stratum (NAS) rejection message indicating that the UE is not allowed in the location area and hence unauthorized to access that FAP. The FAP checks whether the UE is authorized or not by comparing the UE's identity to a list of UE identities pre-configured in the FAP at the time of provisioning.
There are undesired consequences for the UE when a NAS reject is enforced with cause “location area not allowed.” When an LAC is barred, the UE considers that cell is forbidden until a time at which the UE is power cycled or the universal subscriber identity module (USIM) is reinserted. If an LAC is forbidden, any cell broadcasting that particular LAC is considered forbidden for service for the UE.
The undesired effects can be mitigated if the FAP chooses not to reject the UE with a NAS message with cause “location area not allowed.” If the FAP chooses not to reject the UE harshly, i.e., a NAS message with cause “location area not allowed,” the UE returns to the FAP whenever a cell reselection procedure at the UE identifies the FAP as the stronger network. Frequent attempts to access the FAP drains the UE's standby time power quicker and hence creates another undesirable effect.
To counter these adverse effects, existing techniques used in deployed networks include rejecting the unauthorized UEs harshly with a NAS reject with cause “location area not allowed” but at the same time limiting the possibility of such harsh rejects to a minimum. An existing method combats this problem by implementing a “beacon” along with the FAP. A beacon is a “partial” cellular site in that it puts out its own primary scrambling code (PSC), primary common pilot channel (PCPICH), primary common control physical channel (PCCPCH), secondary common control physical channel (SCCPCH), and broadcast channel (BCH). A typical deployment case is that the FAP's serving channel (the channel that serves various UE) is deployed on a dedicated frequency, thus making the femto radio access network (RAN) an inter-frequency deployment with respect to the macro network.
However, the beacon channel, co-located with the FAP's service channel, is deployed on the same frequency as the macro network. That is, the beacon is an intra-frequency neighbor to the macro network. The macro network in its system information only broadcasts (indicates) existence of beacon channels and not the FAP's service channel. Since the FAP's service channel is not broadcast on the macro network, the UE that are camped on the macro network do not measure the inter-frequency neighbor and hence avoid the drastic reduction in standby time associated with measurements of inter-frequency neighbors.
During the measurement and cell reselection procedures, the UE that are camped on the macro network potentially could identify the beacon channel and in some instances identify the beacon channel to be a stronger network for service. When those UE attempt to camp on the beacon by sending a random access attempt, the beacon channel (using one of more possible methods) redirects the UE to the FAP's actual serving channel.
While this method is very attractive, a beacon creates an increase in co-channel interference as the macro network and the beacon channels are deployed on a shared frequency. Due to this reason, any user in a voice or data session with the macro network and coincidentally closer to the beacon channel experiences degraded or even a loss in service. To mitigate this, it has been proposed in the past that the beacon channel remains “on” only for durations of time such that degradation or loss of service is very minimal.
In some embodiments, the beacon is also capable of monitoring an increase in uplink (reverse link) received signal strength indicators (RSSI) to identify whether there is incoming interference. An incoming interference can conversely be assumed to indicate a macro network UE coming close towards the beacon coverage area. In that embodiment, upon detecting an incoming UE, the beacon either switches off completely or reduces its transmitting power just enough to alleviate uplink interference.
If access control is not exercised at the beacon, the UE attempting access at the beacon is redirected (for example, by setting a high cell individual offset via system information block number 11) to the FAP serving cell. An unauthorized UE that gets redirected to the FAP serving cell gets NAS rejected with cause “location area not allowed.” This forces the unauthorized UE to return to the beacon channel and hence becomes unreachable for the duration of the beacon “on” time. In other words, a “page” sent to the unauthorized UE could be lost.